1. Field of the Invention
The present invention relates to a highly sensitive silver halide emulsion, a silver halide photosensitive material, and a photothermographic material, and particularly to a highly sensitive silver halide emulsion using silver halide grains having a high silver iodide content, a silver halide photosensitive material, and a photothermographic material.
2. Description of the Related Art
In recent years, there have been needs for dry development of photographs in the fields of medical diagnoses and printings from the viewpoints of environmental preservation and space saving. Digitalization has progressed in those fields. In a system, image information is input into a computer, stored in the computor, then modified if necessary; a laser image setter or a laser imager at a place where the image is needed accesses the image information through communication, output the image information onto a photosensitive material; the photosensitive material is developed to provide an image on the spot. Such a system is rapidly becoming popular. The photosensitive material needs to be a material on which the image information can be recorded by laser exposure with a high illuminance and on which a clear black image with high resolution and sharpness can be formed. Examples of such digital imaging recording devices are various hard copy systems using pigment or dye such as inkjet printers and electrophotographic devices. The systems have been used as conventional image formation system. However, the systems are not satisfactory in image quality (sharpness, graininess, gradation and color tone), which is important when the system is used to provide medical images, and a recording speed (sensitivity). Therefore, the systems has not been developed to such a degree that they can be used in place of conventional wet-developing silver salt films for medical use.
A heat image-forming system using an organic silver salt has been known. The system uses an image-forming layer in which a reducible silver salt such as organic silver salt, a photosensitive silver halide, and an optional toning agent for controlling color tone of silver are dispersed in the binder matrix.
When a photothermographic material is image-wise exposed and heated to a high temperature (e.g. 80° C. or higher), a black silver image is formed by a redox reaction between a reducing agent and the silver halide or a reducible silver salt that acts as an oxidizing agent. The redox reaction is accelerated by catalysis of the silver halide latent image formed by the exposure, and the black silver image is formed in the exposed region. The photothermographic material has been disclosed in literature, and Fuji Medical Dry Laser Imager FM-DPL started to be sold as a practical system for medical image formation.
Because the image-forming system using an organic silver salt does not have a fixing process, the silver halide remains in the film even after the heat development. The system has two major problems.
One of the problems is that a heat-developed image does not have good storability. Particularly, printout of the image is deteriorated by light. Methods of using silver iodide are known as techniques by which the printout can be improved. The silver iodide causes little printout compared with silver bromide or silver iodobromide with an iodide content of 5 mol % or less. Therefore, there is a possibility that silver iodide can solve the problem. However, since known silver iodide grains have very low sensitivity and cannot be used practically in photothermographic systems. Further, when the silver iodide grains are treated so that the recombination of photoelectrons and positive holes is prevented in order to increase the sensitivity, the excellent printout property is lost.
As described in academic literature, the sensitivity of a silver iodide emulsion can be increased by soaking the silver iodide in an aqueous solution of silver nitrate or a halogen acceptor such as sodium nitrite, pyrogallol, or hydroquinone. The sensitivity can be improved also by sulfur sensitization at a pAg of 7.5. However, the sensitizing effect of the halogen acceptor is subtle and insufficient when used in photothermographic materials.
Compounds having a reducing group and an adsorbent group which adsorbs silver halides as independent groups are disclosed in EP 1308776 A2, etc. as sensitizers for silver halides. However, the compounds cannot provide sufficient sensitivity either, and there are other problems in other properties for practical use.
The other problem is that light scattering by the silver halides remaining in the image-forming system clouds the film, so that the film becomes translucent or opaque and lowers the image quality. To prevent the clouding and solve the problem, as a practical method, fine photosensitive silver halide grains having sizes of 0.08 to 0.15 μm are used and the amount of the fine grains is minimized. However, by this method, the sensitivity is further reduced, the clouding cannot be completely prevented, and the film is hazed by the clouding.
When wet-development is conducted, the material is treated with a fixing solution containing a solvent for silver halides after the development so that the remaining silver halides are removed. Various inorganic or organic compounds capable of forming a complex with a silver ion are known as the solvent for the silver halides.
It has been attempted to apply the fixing process to dry heat-development. For example, it was proposed to add a compound capable of forming a complex with a silver ion to the film so that the silver halides are solubilized (generally referred to as fixing) during the heat-development. However, the method is for silver bromide or silver chlorobromide. Since the method comprises post-heating for fixing at a temperature as high as 155 to 160° C., it is not convenient. Further proposed is a method comprising preparing a fixing sheet containing a compound capable of forming a complex with a silver ion; heat-developing a photothermographic material to form an image; overlapping the photothermographic material on the fixing sheet; and heating them to dissolve and remove the remaining silver halides. However, since the method uses 2 sheets, processes are complicated. The method is practically disadvantageous because it is difficult to maintain the operation stability and waste of the fixing sheet is caused.
In addition to the above methods, a method comprising enclosing a fixing agent for the silver halides in microcapsules and releasing the fixing agent during the heat development is proposed as a fixing method for heat-development. However, in the method, it is difficult to release the fixing agent effectively. A method of using a fixing solution after the heat development is also proposed. However, this method requires a wet process and is not suitable for a completely dry process.
As described above, the known methods for reducing the clouding have many disadvantages, and it is difficult to put the methods into practical use.
It is proposed to use the photothermographic materials as photosensitive materials for photography. When the photosensitive materials for photography is used, an image is recorded on the materials not by scanning exposure of laser or the like based on the image information but by surface exposure. The wet-developing type photosensitive materials have been commonly used as the photosensitive materials for photography such as medical films including direct or indirect X-ray films and mammography films, films for making printing plates, industrial recording films, and photographic films for common cameras. For example, double-sided X-ray photothermographic materials using blue fluorescent screens, photothermographic materials using tabular silver iodobromide grains, and medical photosensitive materials prepared by coating both sides of supports with tabular grains having a high silver chloride content and a major face of (100) are disclosed in patent documents. Further, double-sided photothermographic materials are disclosed also in other patent documents. However, when fine silver halide grains with sizes of 0.1 μm or less are used in known materials, the sensitivity is too low to be used practical photography although the grains do not haze the materials. On the other hand, when silver halide grains with sizes of 0.3 μm or more are used in known materials, the remaining grains haze the materials and deteriorate the printout properties, so that the quality of the formed image is insufficient for practical use.
Wet-developing type photosensitive materials using tabular silver iodide grains is known. However, application of the tabular silver iodide grains to photothermographic materials has not been known. That is because the sensitivity is low as described above, and because methods for effectively sensitizing the grains are not known, and it is technically more difficult to use the grains in the heat-development.
To use a photothermographic material as the photosensitive material for photography, the photothermographic material has to have higher sensitivity and capability to form an image with higher quality including haze.